In electrophotographic applications such as xerography, a charge retentive surface is electrostatically charged and exposed to a light pattern of an original image to be reproduced to selectively discharge the surface in accordance therewith. The resulting pattern of charged and discharged areas on that surface form an electrostatic charge pattern (an electrostatic latent image) conforming to the original image. The latent image is developed by contacting it with a finely divided electrostatically attractable powder referred to as "toner". Toner is held on the image areas by the electrostatic charge on the surface. Thus, a tone image is produced in conformity with a light image of the original being reproduced. The toner image may then be transferred to a substrate (e.g., paper), and the image affixed thereto to form a permanent record of the image to be reproduced. Subsequent to development, excess toner left on the charge retentive surface is cleaned from the surface. The process is well known and useful for light lens copying from an original and printing applications from electronically generated or stored originals, where a charged surface may be imagewise discharged in a variety of ways. Ion projection devices where a charge is imagewise deposited on a charge retentive substrate operate similarly.
Although a preponderance of the toner forming the image is transferred to the paper during the transfer step, some toner invariably remains on the charge retentive surface, it being held thereto by relatively high electrostatic and/or mechanical forces. Additionally, paper fibers, Kaolin and other debris have a tendency to be attracted to the charge retentive surface. It is essential for optimum operation that the toner remaining on the surface be cleaned thoroughly therefrom. Blade cleaning is a highly desirable method for removal of residual toner and debris (hereinafter, collectively referred to as "debris") from a charge retentive surface, because it provides a simple, inexpensive structure compared to the various fiber or magnetic brush cleaners that are well known in the dry electrophotography art. In a typical application, a relatively thin elastomeric cleaning blade member is provided and supported adjacent the charge retentive surface, transverse to the direction of relative movement, with a blade edge chiseling or wiping toner from the surface. Subsequent to release of debris from the surface, the released debris accumulating adjacent the cleaning blade is transported away from the cleaning blade area by a debris transport arrangement or gravity.
It has been noted that after some number of copies are made, paper fibers and other debris tend to become entrapped underneath the blade, between the blade and the charge retentive surface. Unless the debris is dislodged, a copy quality defect tends to occur, characterized by streaks on the copy. Usually, a service call is required to correct the problem.
US-A 3,843,407 to Thorp describes a method of clearing entrapped debris from between the blade and the charge retentive surface by driving a rigid drum photoreceptor in a reverse movement, or back tracking the photoreceptor, to release entrapped debris. To accomplish the back tracking movement, the motor driving the drum is reversed and the reverse motion is transmitted to the drum through a drive shaft and sprocket/chain drive. Generally, back tracking occurs over a selected period of time chosen to given an adequate clearing of debris.
In an electrophotographic device having a flexible belt-type charge retentive member, the driving motor is frictionally coupled to the belt through a driving member, to drive the belt along an endless path typically defined by a set of rolls, one of which is usually a driving member. In such a device there is a natural tendency of the belt to slip. Such belts also commonly have a belt seam, which, if an image is placed over the seam, will cause a copy quality defect. To control operation of the device with respect to the belt, and to assure that imaging does not occur on a belt seam, a hole or indicia is commonly provided on the belt at a fixed position relative to the seam. The hole or indicia is detected, and the imaging operation is started in accordance with the detected belt position.
Typically, to control the amount of back tracking, a drive member would be driven to drive the belt in a reverse direction, with respect to the usual direction, for a selected period of time. However, because slip is likely to occur in that situation, the amount of back tracking is not accurately predictable. Accordingly, subsequent to the back tracking operation, the machine must reacquire the belt position. If the back tracking operation is used relatively frequently, the user may note the delay in device operation required by the machine search for the sensor detectable indicia. Additionally, the slip problem becomes worse as the belt and machine age, so that distance traveled over a short period of time becomes unpredictable. Additionally, the back tracking distance has design limitations. The belt should back track only a selected distance, and not overshoot or exceed that distance, determined by such factors such as the the cleaning region size. However, because of the uncertainty of the back tracking distance, it is not always possible to remain within that distance.
US-A 3,912,390 to van Herten discloses a control circuit for a electrostatic copying machine which regulates the drive system of a photoreceptor belt based on a series of pulsed feedback signals received from an optical sensor which detects the passage of marks notched into a photoreceptor belt. US-A 3,785,730 to Weber et al. discloses a solid state circuit device which controls the movement of a photoreceptor belt by varying the position of image-forming surfaces during successive copy runs. Photodetecting sensors sense the passage of and issue position control signals corresponding to alignment marks on the surface of a photoreceptor belt, to trace out a length of belt defined between two successive marks. US-A 4,577,953 to Narukawa discloses an automatic control system for an electrostatic copying machine comprising a photosensitive sensor which detects cutouts, punched into both sides of a belt-like photosensitive member, and issues signals to a controller based on these cutouts to control the copying process. US-A 4,657,369 to Takeuchi discloses a "smart" electrostatic copying machine comprising an internal computer system which regulates the movement of an endless photoconductive belt based on signals received by a photosensing device which coordinates the position of a photoconductive belt based on the passage and detection of a single notch punched into the belt, as sensed by a photosensor. The notch placed at a predetermined distance from the belt seam, allows the machine belt drive system to avoid placing an image onto the belt seam during the copying process. The references cited herein are all incorporated by reference for their teachings.